Improve support for Firefox on Android

[corbindavenport]

Some possible ideas:

• Pressing the download notification should open the file, at least on mobile Firefox
• Pressing a media link on Firefox for Android seems to allow opening the link with VLC Media Player, maybe this can be used somehow
• The NoPlugin banner doesn't look great on mobile, maybe change this to a notification or remove it completely on mobile

[corbindavenport]

https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Getting_started_with_web-ext#Testing_in_Firefox_for_Android
https://developer.mozilla.org/en-US/Add-ons/WebExtensions/web-ext_command_reference#web-ext_run

[corbindavenport]

The code for the Chrome OS button should be modified to appear on both Android and Chrome OS hosts.